Aramid fibers are highly resistant to heat decomposition, have inherent flame resistance, and are frequently used in working wear for special environments where flame resistance is required. These and other inherent desirable properties of aramid fibers also create difficulties for fiber processing in other areas; specifically, aramids are difficult to dye.
A process for the continuous or semi-continuous dyeing of and simultaneously improving the flame-resistant properties of poly(m-phenyleneisophthalamide) fibers has been described by Cates et al in U.S. Pat. No. 4,759,770. The process includes the use of a fiber swelling agent solution also containing one or more dyes and a flame retardant, the dye and the flame retardant introduced into the fiber while in the swollen state. Suitable swelling agents are dimethylsulfoxide (DMSO), dimethylacetamide (DMAC) and N-methylpyrrolidone (NMP).
Printing of aramid fabrics using a print paste composed of a polar solvent such as DMSO, DMAC or NMP, a dye, water and a print paste thickener is described in Hussamy, U.S. Pat. No. 4,705,527; these print pastes may also include a flame retardant as in Hussamy U.S. Pat. No. 4,706,523. Aramid fabrics printed in a camouflage pattern have specific application for military use where personnel have the potential to be exposed to fire and flame. Fabrics made of highly crystalline aramid fibers, such as DuPont's Nomex.RTM. having high glass transition temperature are difficult to print. The two Hussamy patents noted above describe procedures for obtaining printed aramid fabrics using polar solvents but the processes require some specialized equipment.
An exhaust process for dyeing or simultaneously dyeing and improving the flame resistance of aramid fibers using N-cyclohexyl-2-pyrrolidone (CHP) as a dye carrier under conditions of elevated temperature and optionally elevated pressure is described in PCT/U.S.88/04074 published as WO 89/06292 on Jul. 13, 1989. Although residual CHP remaining on the fibers or fabric is usually removed from the dyed goods prior to further processing, we have found that residual CHP facilitates printing the thus treated aramid fabric with an aqueous print paste. This observation has suggested the application of CHP prior to printing to aramid fabrics in general, regardless of prior processing if any, as a preparatory treatment to printing.
Unlike the highly polar solvents such as DMSO, DMF and NMP which require about 60% concentration in aqueous solution to maintain their swelling/partial solvency of certain aramid fibers, CHP maintains its ability to permeate such fibers in concentrations of only about 5% in aqueous solutions. The ability to work at lower concentrations limits damage organic solvents necessarily cause to aramid fabrics as compared with other aprotic solvents.